It is well known that the output of a loudspeaker should be controlled in such a way that it is not simply driven by any input signal. For example, an important cause of loudspeaker failures is a mechanical defect that arises when the loudspeaker diaphragm is displaced beyond a certain limit, which is usually supplied by the manufacturer. Going beyond this displacement limit either damages the loudspeaker immediately, or can considerably reduce its expected life-time.
It is also well known that the drive signal to the speaker should avoid thermal damage. Loudspeakers are devices which convert electrical energy into acoustical energy. However, much of the electrical power that is applied to the loudspeaker results in heat dissipation, which causes many of the common loudspeaker defects. To prevent thermal damage (permanent or transitory), it is desirable to measure the voice coil temperature of the loudspeaker, and to condition the input in such a way that this temperature does not exceed a certain limit.
One way to estimate the voice coil temperature is to predict its value from the electrical signal that is sent to the loudspeaker using a mathematical model of the loudspeaker using a number of pre-estimated parameters, see e.g. Klippel, W., 2004. Nonlinear Modeling of the Heat Transfer in Loudspeakers Audio Eng. Soc. 52, 3-25.
A different approach is to measure the current and voltage in the voice coil directly and estimate its temperature based on those measurements. This approach is taken in Behler, Gottfried; Spätling, U.; Arimont, T. Measuring the Loudspeaker's Impedance During Operation for the Evaluation of the Voice Coil Temperature in Proceedings of the 98th AES Convention, 5 Paris. Paper number 4001.
From the measured voltage and current, the DC resistance of the loudspeaker, referred to as Re, is determined. The DC resistance is estimated as the average of the real part of the impedance for frequencies in the vicinity of the minimum impedance exceeding the resonant frequency of the loudspeaker. Since the DC resistance depends on the temperature of the voice coil, the temperature can be determined from the DC resistance.
It is known to combine voice coil excursion protection and temperature protection using a digital signal processor which receives the voice coil voltage and current measurements explained above. The cone excursion protection is based on the generic speaker model, taking into account the audio signal provided to the speaker, but tuned by the current measurement and voltage measurement signals. The temperature protection is based on determination of the temperature from the current and voltage measurements.
This invention is concerned in particular with the accuracy of the speaker temperature detection. To protect the speaker well, the speaker temperature needs to be measured with an accuracy of around +/−5° C. The background for this is that the maximum ambient temperature is typically 85° C., and the maximum allowed speaker coil temperature is typically 105° C. (depending on the materials, glues, type of magnets etc.). Thus, there is only a permitted temperature rise of 20° C. above the ambient temperature (85° C.) allowed for the speaker to be able to manage the heat dissipation. As a result, a very accurate temperature measurement for the speaker coil is needed for a practical system.
As explained above, the temperature changes in a speaker can be detected by measuring the dc resistance of the speaker. This resistance is dependant on the temperature as a result of the temperature coefficient of the wire used for the speaker coil.
However, the speaker has an impedance spread caused by the speaker production process. For a typical mobile phone speaker, the nominal resistance is defined as 8 Ohm+/−10%.
The temperature coefficient of a typical copper/silicon compound wire, used to make the speaker coil in mobile phone speakers, is in the order of 0.38%/° C.
This means that a spread of 10% in the nominal dc speaker impedance will already cause an in-accuracy of +/−25° C. in the temperature measurement for the speaker coil.
As the speaker impedance can spread by +/−10% during production of the speaker it is required to take account of the actual connected speaker impedance to be able to measure accurately in real time the speaker coil temperature.